A key factor facilitating the standardization of moral rules within and across groups in human history has been religion. In most societies, fundamental cooperative rules are absolute and unquestionable by virtue of being preserved as divine commands. God, religions promise, will reward adherers and punish transgressors. In a sense this is the ultimate form of indirect reciprocity. Religion reduces the need for policing because believers are to some extent policing themselves through their conscience — to avoid divine, rather than secular, punishment. Of course, people can derive and follow a moral code without, or in spite of, these threats and promises. Nevertheless, the religious approach has proven immensely successful in keeping people in line (although exceptions may spring up). Followers of the same religion can assume that they share a basic code of conduct. If you have the same God, there is no hiding, and you will be judged by the same rules.

The Gap
Thomas Suddendorf